The present invention is directed to a compressor minimum capacity controller which controls the minimum capacity of a compressor. Specifically, the minimum capacity controller is implemented as a software slide valve stop for a helirotor compressor, but is not intended to be so limited. Rather, the invention is intended to be applicable to all variable capacity compressors having minimum capacity limits.
Specifically referring to helirotor or screw compressors, these compressors become less efficient as their load decreases. Increased internal compressor temperatures are the result of this lowered efficiency. At either low evaporator and/or high condenser temperatures, the internal compressor temperatures increase even more. A combination of low load, low evaporator temperatures, and/or high condenser temperatures will increase rotor temperatures such that protection or minimum capacity limiting is necessary to avoid damage to the compressor. Without such a minimum capacity limit, the unloading of the compressor at these conditions leads to overheating of the compressor rotors and the radial expansion or radial growth of the rotors. This radial growth results in a radial rub with the compressor housing, subsequently causing a failure.
In order to avoid compressor failure under extreme conditions, previous systems have installed a physical slide valve stop, also called a puck or a hockey puck, to set a minimum load based on the anticipated operating conditions of the customer's applications. However and unfortunately, a single mechanical stop is not optimum for all operating conditions, a single mechanical stop does not compensate for changes in the system design after original installation, and a single mechanical stop does not compensate for operator error in sizing the mechanical stop. A minimum capacity limit controller is desirable that will automatically adjust the compressor at minimum load for contemporaneous conditions, allowing the screw compressor system to adapt to variable operating conditions.
The simplest approach would be to measure the rotor temperature itself and establish a minimum capacity limit based upon that measured temperature. However, measuring the rotor temperature directly is difficult to implement without significant cost and operating efficiency penalties. A substitute measure for rotor temperature, such as compressor refrigerant discharge temperature is preferred. The present invention proposes a minimum capacity limit control that will control the compressor refrigerant temperature by limiting unloading, suspending loading, or initiated forced loading of the compressor responsive to a measured condition where the measured condition is directly related to rotor temperature.